1. Field of the Invention
The invention relates to an open-loop and closed-loop control method for a self-commutating three-point converter that is fed from a DC voltage intermediate circuit and has active clamped switches. Typically, such three-point converters have two series-connected main switches/inverse diodes between each DC voltage connection and each load connection, with the common junction point of the two inner main switches forming the load connection, and with an active clamped switch with an inverse diode being connected between each common junction point of an inner main switch and an outer main switch and the center tap of the DC voltage intermediate circuit, as a result of which two possible paths are formed for connecting a load connection to the center tap. The invention further relates to an apparatus for accomplishing the method. Converters such as these may be used both as self-commutated rectifiers and as self-commutated inverters. They are primarily used in medium-power and high-power electrical drives.
The topology of the self-commuted, diode-clamped three-point converter on the DC intermediate circuit (three-point NPC converter) is generally known. It is also used industrially for fields of application such as high-power industrial or traction drives (medium-voltage drives). In this case, insulated gate bipolar transistor (IGBT) modules with an integrated inverse diode are used as main switches. For reasons of modularity, simplification of the mechanical construction, or else in order to ensure that the blocking voltage is shared uniformly when semiconductor components are connected in series in converters such as these, IGBT modules are also frequently installed as NPC switches (referred to in the following text as active NPC switches or active neutral-point clamped switches, or active clamped switches) instead of neutral-point clamped diodes (NPC diodes). These IGBTs are in this case either placed in the xe2x80x9coffxe2x80x9d state by short-circuiting the gate-emitter path, or else are operated in the active range in order to control the blocking voltage distribution, while the integrated inverse diode carries out the function of the NPC diode.
FIG. 1 shows a generally known, self-commutated three-point converter, fitted with NPC switches, of this type on the DC intermediate circuit, or three-point NPC converter, for short. An outer main switch T1U, T1V, or T1Wxe2x80x94referred to in general form by T1 in the following textxe2x80x94and an inner main switch T2U, T2V, or T2Wxe2x80x94also referred to in general form in the following text as T2xe2x80x94are respectively connected in series between the positive DC voltage connection and the three load connections, with a respective inverse diode D1U, D1V, or D1Wxe2x80x94also referred to in general form in the following text as D1xe2x80x94being connected back-to-back in parallel with each outer main switch T1U or T1V or T1W, respectively, and a respective inverse diode D2U, D2V, or D2Wxe2x80x94also referred to in general form in the following text as D2xe2x80x94being connected back-to-back in parallel with each respective inner main switch T2U, T2V, or T2W.
A respective outer main switch T4U, T4V, or T4Wxe2x80x94also referred to in general form in the following text as T4xe2x80x94and an inner respective main switch T3U, T3V, or T3Wxe2x80x94also referred to in general form in the following text as T3xe2x80x94are connected in series between the negative DC voltage connection and the three load connections, with a respective inverse diode D4U, D4V, or D4Wxe2x80x94also referred to in general form in the following text as D4xe2x80x94being connected back-to-back in parallel with each respective outer main switch T4U, T4V, or T4W, and a respective inverse diode D3U, D3V, or D3Wxe2x80x94also referred to in general form in the following text as D3xe2x80x94being connected back-to-back in parallel with each respective inner main switch T3U, T3V, or T3W. The load-side phase currents (load currents) are annotated iphU, iphV, and iphW.
The common junction point of T1U, D1U, T2U, and D2U is connected via an active NPC switch T5U with a back-to-back parallel-connected inverse diode D5U to the center tap of the DC intermediate circuit. The common junction point of T1V, D1V, T2V, and D2V is connected in the same way via an active NPC switch T5V with a back-to-back parallel-connected inverse diode D5V to the center tap of the DC intermediate circuit. In the same way, the common junction point of T1W, D1W, T2W, and D2W is connected via an active NPC switch T5W with a back-to-back parallel-connected inverse diode D5W to the center tap of the DC voltage intermediate circuit. The active NPC switches T5U, T5V, T5W are also referred to in general form in the following text as T5. The inverse diodes D5U, D5V, D5W are also referred to in the following text as D5.
The center tap is connected via two capacitors with the same capacitance to the two DC voltage connections. The voltage across each of the capacitors is Vdc/2 (half the intermediate circuit voltage).
The common junction point of T3U, D3U, T4U, and D4U is connected via an active NPC switch T6U with a back-to-back parallel-connected inverse diode D6U to the center tap of the DC voltage intermediate circuit. The common junction point of T3V, D3V, T4V, and D4V is connected in the same way via an active NPC switch T6V with a back-to-back parallel-connected inverse diode D6V to the center tap of the DC voltage intermediate circuit. In the same way, the common junction point of T3W, D3W, T4W, and D4W is connected via an active NPC switch T6W with a back-to-back parallel-connected inverse diode D6W to the center tap of the DC voltage intermediate circuit. The active NPC switches T6U, T6V, T6W are also referred to in general form in the following text as T6. The inverse diodes D6U, D6V, D6W are also referred to in the following text as D6.
An investigation into diode-clamped three-point NPC converters with sinusoidal modulation shows that the thermal configuration of these converters is governed by four critical operating points, which are quoted in the following Table I. At each of these four critical operating points, the phase current (load current) and hence the output power from the converter is limited by the maximum permissible losses in those power semiconductors which are most heavily loaded at this critical operating point. All the other semiconductors reach only a lower boundary layer temperature at the respective critical operating points. Since the maximum losses and the maximum boundary layer temperatures of the individual semiconductors reach comparable values at the operating points that are critical for them, all the components must be replaced by larger components if the output power of the converter is to be increased.
An additional critical operating point when using converters in electrical drive systems, particularly those with synchronous machines, is the starting or stopping of the drive. This situation is characterized by a very low output frequency from the converter, down to zero Hertz, and a low modulation level M. The phase current (load current) is in this case limited by the losses in the NPC diodes, which corresponds to case 2 in Table I below. Due to the low output frequency, one phase may be loaded with the peak value of the load current for a certain time period, this being sufficient to reach the thermally steady state. The achievable load current is thus reduced considerably in comparison to operation at high output frequencies. Although this problem can be minimized by reducing the switching frequency while stopping, a reduction in the load current with respect to the rated current whilst stopping cannot be avoided in conventional medium-voltage drives. Applications such as hot and cold rolling mills typically demand 200% load torque and hence twice the load current when the drive is being stopped, however. In consequence, satisfaction of this condition leads in a disadvantageous manner to considerable overdesign of the three-point NPC converter.
With reference to what has been stated above, the non-uniform distribution of the losses between the individual semiconductor components is a major disadvantage of diode-clamped three-point NPC converters and of three-point converters that are operated like diode-clamped three-point NPC converters and have active NPC switches. This also means that the utilization level of the semiconductor components, in particular of the inner main switches, is relatively low. Furthermore, it should be stated that the capabilities of the active NPC switches (with inverse diodes), which are frequently installed instead of the NPC diodes, for influencing the distribution of the losses in the semiconductor components have so far not been actively made use of.
It is accordingly an object of the invention to provide an open-loop and closed-loop control method for a three-point converter with active clamped switches, and an apparatus for this purpose that overcome the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and that make the loss distribution between the semiconductor components of a phase module in the converter uniform at all operating points, even while stopping the drive.
With the foregoing and other objects in view, there is provided, in accordance with the invention, an open-loop and closed-loop control method. The method involves providing a three-point converter for connecting a DC voltage intermediate circuit having a positive DC voltage connection, a center tap, and a negative DC voltage to three load connections. The next step is, irrespective of a direction of a load current, connecting at least one of the two active clamped switches to the center tap together with at least one of the inner main switches for connecting to one of the load connections, in order to carry the current deliberately through at least one of the upper path and the lower path, to the center tap during a null state.
With the objects of the invention in view, there is also provided an open-loop and closed-loop control apparatus. The apparatus includes a three-point converter for connecting a DC voltage intermediate circuit having a positive DC voltage connection, a center tap, and a negative DC voltage to three load connections. The three-point converter includes two series-connected main switches/inverse diodes connected between each of the DC voltage connections and each of the load connections, and two inner main switches sharing a common junction point, the common junction point forming one of the load connections, and an active clamped switch with an inverse diode connected between each common junction point of an inner main switch and an outer main switch and the center tap to form an upper path and a lower path for connecting respective load connections to the center tap. The switches and diodes are semiconductors having phase currents and boundary layer temperatures. A modulator produces switching state commands. A temperature regulator and automatic drive device can form control signals for the semiconductor switches from the switching state commands of the modulator, the phase currents, and the boundary layer temperatures of the semiconductors.
The advantages that can be achieved by the invention are, in particular, making the loss distribution uniform between the NPC switches and the inner switches in cases 2 and 4 (motor or generator operation with a very low modulation level) according to Table I:
considerably reducing the complexity of semiconductors overall while maintaining the output power of the converter by using smaller semiconductors as inner main switches and active NPC switches or, alternatively,
allowing the power to be reduced to a lesser extent while stopping, while keeping the installed switch rating constant.
Furthermore, the reduction in the load on the outer main switches and diodes at the critical operating points 1 and 3 (motor or generator operation with maximum modulation level) according to Table I above is achieved at the expense of the inner main switches and diodes and hencexe2x80x94associated with the likewise mentioned unification of the loss distribution between the NPC switches and the inner switches in cases 2 and 4xe2x80x94an increase in the output power of the converter (increase in the power yield) or an increase in the switching frequency is achieved without increased complexity in terms of semiconductor components.
Other features that are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a open-loop and closed-loop control method for a three-point converter with active clamped switches, as well as an apparatus for this purpose, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.